...I am willing to change batteries but don't do brain surgery.
+1
When I bought my used RV, I spent a lot of time going through everything. I had read that there were many RV fires each year due to electrical shorts or propane line problems.
So, I carefully checked the big heavy gauge wiring around the house batteries - "house" meaning the batteries that are for living and not the starter battery for the engine. While the batteries were being under charge by the power converter, I measured the voltage drop across each connection, each crimp terminal, the isolator relay, etc... to make sure there was good contact.
At one crimp terminal, the voltage drop was a bit high. It was in the tens of millivolts compared to other places that were in the millivolts.
Hmm... I felt that terminal lug, and it was a bit warm. Of course the contact had higher than normal resistance. The cable looked a bit corroded. Hmm... I wiggled it a bit to see if the voltage drop changed. Yes, of course...
And then, the darn big #2 cable in my hand
came off its lug. Good lord! The crimp terminal had worked itself loose, and the cable end was barely hanging there in the socket. The hair on the back of my neck raised when I thought of what could happen. Holy mackerel! Imagine if this thing came off and shorted its end against the steel enclosure holding the batteries while I was driving. Talk about unintentional arc welding!
So, I spent time to rip out all these chintzy cables with crimp terminals, put in all new ones with soldered ends, and put fuses in there too. The house batteries and the engine battery were also linked via a battery isolator relay. This was a long #2 gauge cable snaking through the chassis and the engine compartment, and there were no fuses at all to protect against fire should the cable rub against the chassis and short itself out. So, I put in big fuses, one on each end.